In recent years, liquid crystal displays have come into wide use in home TVs, computers, picturephones, and so forth. Such liquid crystal displays are mostly backlight displays. Since the reproducibility of image is needed especially in the fields of printing, medical care, etc., it is customary in the art to monitor the quantity of light from the backlight by a photodetector placed in the rear of the liquid crystal display and control the backlight accordingly. Since the light transmission characteristic of the liquid crystal panel greatly varies nonlinearly with working temperature and due to age deterioration, there is a limit to enhancement of the gray scale from the practical point of view. Further, there have also been developed liquid crystal displays provided with means for monitoring a user image and illuminance in the use environment, but in many cases an image sensor or photodetector is disposed behind or beside the display screen, or a manual sensor is mounted on the display screen for manual calibration.
However, in the case of monitoring the quantity of light from the backlight by the photodetector mounted in the liquid crystal display as in the prior art, there arises a problem that no appropriate monitoring of the quantity of light is possible since the amount of light reflected rearwardly of the liquid crystal panel differs depending on whether the liquid crystal is transparent to light or not. Furthermore, depending on whether ambient brightness is high or low, the incidence of external light from the front of the liquid crystal panel disturbs monitoring by the photodetector, sometimes hindering appropriate luminance control of the backlight. It is also necessary to monitor the illuminance in the use environment. The photodetector or image sensor mounted on the front of the liquid crystal display for monitoring the illuminance and the user image in the use environment ruins the appearance of the display. Besides, since the optical transfer characteristic of the liquid crystal panel changes with aging or temperature, reproducibility of the gray scale of image may sometimes lower even if the quantity of light from the backlight is controlled to be constant; hence, it is to be wished that a high-precision calibration scheme be established.